A drive device for an omni-directional vehicle that can freely move about on a floor is known, for instance, from Japanese patent No. 3820239 (patent document 1). In particular, reference should be made to the third embodiment illustrated in FIGS. 17 and 18 of this patent document. The illustrated drive device includes a rotatively driven wheel and a plurality of shafts attached to the outer periphery of the driven wheel and each extending in a tangential direction. A barrel shaped roller divided into a plurality of parts is supported by each shaft so as to be rotatable around the shaft, and each barrel shaped roller is engaged to the adjacent rollers so as to transmit the rotational movements of the barrel shaped rollers around the corresponding shaft to one another. One of the barrel shaped rollers is rotatively actuated around the corresponding shaft by an electric motor incorporated in the driven wheel.
In this drive device, the rotation of the driven wheel applies a traction in a fore-and-aft direction (or a direction of the rotation of the driven wheel) to the ground contact surface via each barrel shaped roller, and the rotation of the individual barrel shaped rollers applies a traction in a lateral direction (or a direction of the rotation of the barrel-shaped rollers) via the ground contact surfaces of the barrel shaped rollers. Therefore, by suitably selecting the contributions of the fore-and-aft traction and lateral traction, the vehicle can travel in any desired direction.
In such a friction drive vehicle, it is essential for an efficient transmission of power from a drive roller to a driven roller that the drive roller and driven roller engage each other with an optimum contact pressure. If the contact pressure is too small, a slippage arises between the drive roller and driven roller, and this impairs the power transmission efficiency. If the contact pressure is too great, the frictional resistance produced between the drive roller and driven roller becomes excessive, and this also impairs the power transmission efficiency. Additionally, an excessive contact pressure adversely affects the bearings for the drive roller and driven roller.
Also, the optimum contact pressure may vary depending on the load of the vehicle such as the weight of the vehicle occupant. A greater load requires a greater traction, and the contact pressure must be increased in a corresponding manner so as to allow the required power to be transmitted from the drive roller to driven roller.